Paved surfaces such as concrete and asphalt roads are today marked with stripes to denote vehicular lanes and stop lines. The material used for such markings is typically an alkyd, epoxy or hydrocarbon thermoplastic which is applied in a molten form to the paved surface and then allowed to set and bond. Mobile applicators used in applying such thermoplastic materials are commonly referred to as road stripers. Some stripers are hand propelled while others are self propelled. As exemplified by the striper disclosed in U.S. Pat. No. 3,362,399 these applicators typically have a carriage or housing mounted upon a set of wheels in which a reservoir is housed and heated for containing a supply of the marking material in its molten state. The material is dispensed from the reservoir and onto a paved surface beneath the carriage through an extruder or sprayer. For efficiency of application the thermoplastic material is purchased in batch, solidified form, typically in the shape of a rectangular block or slab. Though the slabs, which sometimes are referred to as cakes, can be placed directly into the heated reservoir for melt down, it is more efficient to melt them in a preheater and then to channel the melt into the reservoir which may already contains a supply of the marking material in its molten state.
Road striper preheaters have heretofore typically comprised an upright, rectangular receiver or holder mounted vertically within a casing with the space between the receiver and casing providing a hot gas flow chamber. One or more fire tubes extends into this chamber from a supply tank of combustable fuel such as propane. By dropping a slab of the thermoplastic marking material into the receiver from ambience, and by igniting a flow of gas emitted through orifices in the fire tubes, heat generated and circulated through the gas flow chamber is transferred to the walls of the receiver and then to the slab, thereby causing it to melt. The flow of molten material is channelled out of an open bottom of the receiver into the reservoir where it is maintained in a molten state by other propane burner means until applied by an extruder or sprayer onto the surface of the roadway over which the applicator is moved.
Though preheaters of the type just described have functioned satisfactorily they do take significant periods of time to complete the melting of slabs. Such periods can provide loss times thereby restricting the efficiency of marking operations both from direct application time loss as well as from an excessive consumption of burner fuel.